1. Field of the Invention
The present invention relates to a method of manufacturing a complementary metal-oxide semiconductor (CMOS) device. More particularly, the present invention relates to a method of manufacturing a CMOS sensor having a silicon nitride layer as an Anti-Reflection layer.
2. Description of the Related Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting light into an electrical signal that represents the intensity of the energy. The applications of the CCDs include monitor, transcription machines and cameras. Although CCDs have many strengths, CCDs also suffer from high costs and the limitation of the CCDs' volume. To overcome the weaknesses of CCDs and reduce costs, dimension and energy consumption, a CMOS photo diode device is developed. Because a CMOS photo diode device can be produced using conventional technique, costs and the volume of the sensor can be reduced. The applications of CMOS photo diodes include PC cameras, digital cameras etc.
A photo diode based on the theorem of a P-N junction can convert light into an electrical signal. Before energy in the form of photons strikes the photo diode, there is an electric field in the P-N junction. The electrons in N region do not diffuse forward to P region and the holes in P region do not diffuse forward to N region. When enough light strikes the photo diode, the light creates a number of electron-hole pairs. The electrons and the holes diffuse forward to the P-N junction. While the electrons and the holes reach the P-N junction as a result of the effect of the inner electric field across the junction, the electrons flow to the N region and the holes flow to the P region. Thus a current is induced between the P-N junction electrodes. Ideally, a photo diode in the dark is open-circuit. In other words there is no current induced by light while photo diode is in the dark.
FIG. 1 is a schematic, cross-sectional view of a portion of a semiconductor device showing a conventional CMOS sensor. In FIG. 1, the conventional CMOS sensor includes a P-type substrate 100, a field oxide layer 104, a P-type well 110, a gate structure 120, an N-type source/drain region 122, an N-type sensor region 124, an depletion region 126, and a borophosphosilicate glass/silicon nitride glass dielectric layer.
When a light beam 140 passes through the depletion region 126 which works as an P-N junction, the depletion region 126 is excited and a number of electron-hole pairs are created. Thus the light is converted into an electric signal. Because the light beam 140 strikes the depletion region 126 just once, the effective interaction length is short. The intensity of electric signal is small to result in a low contrast ratio of brightness. That is, the sensitivity is low. Moreover, the light beam 140 passing though the depletion region 126 is absorbed by the substrate 100. This produces unnecessary current and induces substrate leakage.